JP5126479B2 - Quantum key distribution system and receiving apparatus - Google Patents

Description

  The present invention relates to a quantum cryptography system, and more particularly to a quantum key distribution system that shares an encryption secret key by optical fiber communication.

  With the explosive spread of the Internet and the commercialization of electronic commerce, there is an increasing social need for cryptographic technologies such as maintaining confidentiality of communications, preventing tampering, and personal authentication. Currently, a common key method such as DES encryption and a public key method such as RSA encryption are widely used. However, these are based on “computational safety”. In other words, current cryptosystems are constantly threatened by advances in computer hardware and decryption algorithms. Therefore, in particular, in fields where extremely high security is required, such as transactions between banks and military / diplomatic information, it is expected that the impact will be great if practically secure cryptographic methods are put into practical use.

  There is a one-time pad method as an encryption method whose unconditional security is proved by information theory. The one-time pad method is characterized in that an encryption key having the same length as the communication text is used, and the encryption key is discarded once. In Non-Patent Document 1, Bennett et al. Proposed for the first time a specific protocol that is widely known as the BB84 protocol and that securely distributes a cryptographic private key used in the one-time pad method. As a result, research on quantum cryptography has become active. In quantum cryptography, the laws of physics guarantee the security of cryptography, so it is possible to guarantee the ultimate security that does not depend on the limits of computer capabilities. In quantum cryptography devices that are currently being studied a lot, 1-bit information is encoded and transmitted in a single photon state. This is because photons are more resistant to environmental disturbances than other quantum systems, and at the same time, long-distance encryption key distribution can be expected by utilizing existing optical fiber communication technology.

  As described in Non-Patent Document 1, in a quantum cryptography device whose security has been theoretically proved, two distinct states of a quantum mechanical two-degree-of-freedom system and a conjugate state (a superposition thereof) The secret key is transmitted securely using the combined state. An eavesdropping act disturbs the quantum mechanical state, and the protocol is designed so that the amount of leaked information can be estimated from errors in the data of authorized senders and receivers. The quantum state used for such information communication is often called quantum information. A quantum mechanical two-degree-of-freedom system carrying quantum information is called a qubit, which is mathematically equivalent to a spin 1/2 system. Hereinafter, the prior art will be described in the case where the physical system as a carrier is a photon.

  A related art of an encryption key distribution apparatus that uses photons as qubit carriers and optical fibers as transmission lines for long-distance transmission related to the present invention will be described below. Non-patent documents 2 to 4 describe the quantum cryptography device using photons in detail. Non-Patent Document 1 proposes the implementation of a quantum cryptography device called polarization coding that encodes information into two polarization states that a photon can have. However, since polarization coding requires real-time control and compensation of polarization rotation in the transmission line, it is not often used as an implementation method for a long-distance encryption key distribution system using an optical fiber as a transmission line.

  As a long-distance encryption key distribution system, Bennet et al. Have proposed and realized an implementation of a quantum encryption device called phase coding that encodes information in a relative phase between two weak light pulses.

  FIG. 3 shows a typical implementation example of a quantum cryptography device based on phase coding described in Non-Patent Documents 2 to 4. In this quantum cryptography apparatus, an optical interferometer having a structure in which two asymmetric Mach-Zehnder interferometers are connected in series by an optical fiber transmission line is used. By making a weak short light pulse generated by a weak laser light source 301 provided on the transmission unit (Alice) side incident on an asymmetric Mach-Zehnder interferometer 302 on the transmission side, the long and short optical path on the optical fiber transmission line 303 A coherent duplex optical pulse 308 spatially separated by the difference is prepared. Here, the term “coherent” means that the relative phase is clearly defined between two pulses of the double pulse by the asymmetric Mach-Zehnder interferometer 302 in which the long and short optical path differences are clearly defined.

  Although the double light pulse 308 is disturbed during transmission on the optical fiber transmission line 303, the relative phase relationship and the polarization plane relationship are preserved. The asymmetric Mach-Zehnder interferometer 304 on the receiving unit (Bob) side converts the duplex light pulse 308 into a triple light pulse 309 and outputs it to the two downstream output ports. The photon detector 305 connected to the two output ports determines whether the photon contained in the center optical pulse of the triplet optical pulse 309 output to the two output ports of the asymmetric Mach-Zehnder interferometer 304 is 0 or 1. Identify and record. Among the triplet optical pulses 309, the central optical pulse includes an optical pulse that has passed through the long part of the asymmetric Mach-Zehnder interferometer 302 on the transmitting side and passed through the short part on the asymmetric Mach-Zehnder interferometer 304 on the receiving side. The light pulse that has passed through the short part of the asymmetric Mach-Zehnder interferometer 302 on the transmitter side and passed through the long part on the asymmetric Mach-Zehnder interferometer 304 on the receiver side contributes, and two outputs are generated by interference of these two contributions. The intensity ratio to the port depends on the optical delay (relative phase) of the duplex light pulse 308 in a sinusoidal function. In this optical interference system, by modulating the optical delay (relative phase) of the double light pulse 308, encryption key distribution based on the principle of quantum cryptography can be performed.

  For this purpose, while an optical pulse passes through the asymmetric Mach-Zehnder interferometer 302, the phase modulator 306 included therein performs four-level phase modulation of {0, π / 2, π, 3π / 2}. . On the other hand, while the duplex optical pulse 308 transmitted through the optical fiber transmission line 303 passes through the asymmetric Mach-Zehnder interferometer 304, the phase modulator 307 included therein includes {0, π / 2} binary phase modulation. I do. By appropriately adjusting the optical delay in the asymmetric Mach-Zehnder interferometers 302 and 304, the quantum key distribution protocol using the non-orthogonal four states proposed in Non-Patent Document 1 can be executed to perform secure key distribution. Is possible.

  A quantum cryptography device based on phase coding has a merit that compatibility with an optical fiber transmission line is good and long-distance key distribution is possible. On the other hand, there is a problem that the relative optical delay of the asymmetric Mach-Zehnder interferometers possessed by each of the transmitter and receiver must be maintained with the same accuracy as the optical wavelength. Since the optical delay of the asymmetric Mach-Zehnder interferometers distributed in these transmitters and receivers fluctuates and drifts independently due to temperature changes and other causes, the optical interference effect is easily lost. In order to solve this problem, an active control device that measures the relative optical delay change of both interferometers with a measuring device and feeds back the measurement result to keep the relative optical delay constant is required. Such a measuring apparatus not only complicates the system itself, but the reference light used for the measurement increases system noise and causes the performance degradation of the quantum cryptography apparatus.

  In order to solve such problems, as disclosed in Non-Patent Documents 5 to 7, quantum cryptography devices applying a planar lightwave circuit (PLC) technology have been proposed and developed. By fabricating an asymmetric Mach-Zehnder interferometer with an optical waveguide formed by patterning on a silicon substrate, a stable optical interferometer that is not affected by disturbance can be realized only by passive control such as temperature control. This has the advantage that a low noise system can be constructed.

  However, in the case of mounting using a PLC, it is not easy in the state of the art to manufacture a low-loss asymmetric Mach-Zehnder interferometer including a phase modulator as described above. Even if the increase in cost is not a problem, an increase in the optical loss of the device is an unacceptable problem because it directly leads to a deterioration in the performance of the quantum cryptography device using weak light as an information carrier.

  In order to solve this problem, a quantum cryptography device as shown in FIG. 4 in which a phase modulator is disposed outside an asymmetric Mach-Zehnder interferometer has been proposed and developed.

  In this quantum cryptography apparatus, a weak short light pulse generated by a weak laser light source 401 provided on the transmission unit (Alice) side is incident on an asymmetric Mach-Zehnder interferometer 402 configured by a PLC on the transmission unit side. Thus, a coherent duplex optical pulse 409 is prepared on the optical fiber transmission line 403 by being spatially separated by the difference between the long and short optical paths. The double light pulse 409 is transmitted on the optical fiber transmission line 403. The asymmetric Mach-Zehnder interferometer 404 configured by a PLC on the receiving unit (Bob) side converts the duplex light pulse 409 into a triple light pulse 410 and outputs it to two downstream output ports. The photon detector 405 identifies and records the presence or absence of a photon contained in the center optical pulse of the triplet optical pulse 410 output to the two output ports of the asymmetric Mach-Zehnder interferometer 404.

  A pulse-like modulation signal is applied to the phase modulators 406 and 407 inserted and connected in series downstream of the asymmetric Mach-Zehnder interferometer 402 on the transmitter side in synchronization with the passage of the double optical pulse 409 of each phase modulator. To do. As a result, the four-valued phase modulation of {0, π / 2, π, 3π / 2} is selectively given to one pulse of the double light pulse 409, so that the optical delay (relative phase) of the double light pulse 409 is obtained. ) Is given a four-value modulation. On the other hand, a pulsed modulation signal is applied to the phase modulator 408 inserted and connected in series upstream of the asymmetric Mach-Zehnder interferometer 404 on the receiving side in synchronization with the passage of the double light pulse 409. As a result, binary phase modulation of {0, π / 2} is selectively given to one pulse of the duplex optical pulse 409, and thus binary modulation is applied to the optical delay (relative phase) of the duplex optical pulse 409. give. By adjusting the optical delay in the asymmetric Mach-Zehnder interferometers 402 and 404, the quantum encryption key distribution protocol using the non-orthogonal four states proposed in Non-Patent Document 1 is executed in the same manner as the quantum encryption apparatus of FIG. Key distribution is possible.

International Conference on IEEE Computers, Systems, and Signal Processing by Bennett and Brassard (IEEE Int. Conf. On Computers, Systems, and Signal Processing, Bangalore, India, p. 175 (1984)) Zbinden et al., “Experimental Quantum Cryptography”, “INTRODUCTION TO QUANTUM COMPUTATION AND INFORMATION (edited by Lo et al.)” (World Scientific, 1998), 120 pages Ekert et al., “Quantum Cryptography”, “The Physics of Quantum Information (edited by Bouwmeester et al.)” (Springer, 2000), 15 pages Gizan et al., “Quantum Cryptography” Review of Modern Physics, Rev. Mod. Phys., 74 (2002), pages 145-195 Nambu et al. “BB84 Quantum Key Distribution System Based on Silica-Based Planar Lightwave Circuits” Japan Journal of Applied Physics, No. 43 (published in 2004), L1109 pages Kimura et al. "Single-photon Interference over 150 km Transmission Using Silica-based Integrated-optic Interferometers for Quantum Cryptography" Japan Journal of Applied Physics (Jpn J. Appl. Phys.), No. 43 (published in 2004) , L1217 page Nanbu et al. “One-way Quantum Key Distribution System based on Planar Lightwave Circuit” Japan Journal of Applied Phys., No. 45, Volume 6A (published in 2006), pages 5344-5348

  The first problem of the above technique is the optical loss of the phase modulator on the receiving unit side. The phase modulator typically has a loss of about 3 dB, which directly leads to a decrease in the encryption key generation rate.

  The second problem is that high-speed control of the phase modulator is required on both the transmission unit side and the reception unit side. The long and short optical path difference of an asymmetric Mach-Zehnder interferometer is typically about 5 nanoseconds, and high-speed modulation on the order of 1 nanosecond is required to perform phase modulation on only one of the two series of optical pulses. In addition, in order to apply this high-speed modulation in synchronization with the passage of the optical pulse, highly accurate signal synchronization is also required.

  An object of the present invention is to improve the encryption key generation rate by making it possible to eliminate the phase modulator from the receiving unit side.

  Another object of the present invention is to provide a quantum key distribution system that requires a simpler configuration than conventional systems.

  The transmitting apparatus in the quantum key distribution system according to the present invention is characterized in that two phase modulators are connected in parallel and used in order to generate the non-orthogonal four states necessary for the BB84 protocol.

  Specifically, the transmitter includes a laser light source that generates an optical pulse, an optical delay circuit that receives the optical pulse and generates a coherent double optical pulse, such as an asymmetric Mach-Zehnder interferometer, and the optical delay circuit. A first coupler for branching the optical path from the first coupler, first and second phase modulators connected to each output port of the first coupler, and the first and second phases. A second coupler that couples the output of the modulator to one optical path, wherein the first phase modulator performs phase modulation of {0 °, 90 °} on the incident optical pulse, and a second phase The modulator is characterized in that encoding is performed by performing phase modulation of {0 °, 180 °} on the incident optical pulse.

  On the other hand, the receiving device in the quantum key distribution system according to the present invention does not require a phase modulator, and an optical delay circuit for analyzing the relative phase of the duplex optical pulse from the transmitting device, for example, an asymmetric Mach-Zehnder interferometer, And an optical system for analyzing the arrival time of the light pulse.

  In the receiving device, the optical pulse from the transmitting device is branched into two by a coupler, and the branched one optical pulse is sent to the asymmetric Mach-Zehnder interferometer on the receiving device side. The optical system may be incident on each of the optical systems, or alternatively, an optical pulse from the transmitter may be incident on the asymmetric Mach-Zehnder interferometer on the receiver and branched into the asymmetric Mach-Zehnder interferometer. An optical pulse branched by connecting a coupler may be incident on the optical system.

  According to the present invention, there is also provided a quantum key distribution system including the above transmission device and the above reception device.

  According to the present invention, it is possible to eliminate the phase modulator from the receiving apparatus, so that the encryption key generation rate can be improved. Moreover, since the control of the phase modulator in the receiving apparatus is not necessary, a quantum key distribution system having a simpler configuration than the conventional quantum key distribution system can be provided.

[Concept of Invention]
Before describing an embodiment of a quantum key distribution system according to the present invention, the concept of the present invention will be described.

  In the transmission apparatus in the quantum key distribution system according to the present invention, first, an optical pulse generated from a weak laser light source is converted into a double optical pulse by an optical delay circuit such as an asymmetric Mach-Zehnder interferometer. Next, the double light pulses are incident on two phase modulators connected in parallel, and their output lights are combined and transmitted to a receiving device through an optical fiber communication path.

  A phase modulator connected in parallel is a system in which a dual optical pulse as input light is branched into two optical paths, one phase modulator is arranged in each optical path, and then the two optical paths are combined again. It is. The input / output relationship of the two phase modulators connected in parallel can be expressed by the following equation (1).

In the above equation, Ein and Eout represent the complex electric field amplitude of the input / output light, and φ ₁ and φ ₂ represent the phase shift amounts by the respective phase modulators. In the present invention, each phase modulation is a binary modulation, and values of φ ₁ = {0 °, 90 °} and φ ₂ = {0 °, 180 °} are employed. The intensities and phases of the four states output in this case are as shown in Table 1 below.

  By using two phase modulators connected in parallel and individually modulating each optical pulse of the branched duplex optical pulse, it is possible to generate the non-orthogonal four states necessary for the BB84 protocol. it can.

In the first state, the modulation of {φ ₁ , φ ₂ } = {0 °, 0 °} is applied to the front pulse of the double light pulse, and {φ ₁ , φ ₂ } = {0 °, 180 ° is applied to the rear pulse. } Are respectively applied. As a result, it is possible to generate a state of only the previous pulse having an intensity of 1. In the second state, the same modulation as that in the first state is performed in the reverse order, so that a state having only a post-pulse having an intensity of 1 is generated. Here, for convenience, these two states are referred to as “two states belonging to the Z base”, and bit 0 and bit 1 are assigned to each. Since these two states are orthogonal states, they can be reliably identified by an appropriate measurement method.

In the third state, the modulation of {φ ₁ , φ ₂ } = {90 °, 0 °} is applied to the front pulse of the double light pulse, and {φ ₁ , φ ₂ } = {90 °, 180 ° is applied to the rear pulse. } Are respectively applied. As a result, it is possible to generate a double light pulse having an intensity of 1/2 and a relative phase of the front and rear pulses shifted by + 90 °. In the fourth state, the same modulation as in the third state is performed in the reverse order to generate a double light pulse whose intensity is ½ and the relative phase of the front and rear pulses is shifted by −90 °. These two states are called “two states belonging to the Y base”, and bits 0 and 1 are assigned again. Since these two states are also orthogonal states, they can be identified by an appropriate measurement method. In addition, since the state belonging to the Z base and the state belonging to the Y base are not orthogonal, it is impossible to identify without any error using any measurement method.

  As described above, the non-orthogonal four states necessary for the BB84 protocol can be generated by the two phase modulators connected in parallel with the double optical pulse.

  The receiver in the quantum key distribution system according to the present invention uses an asymmetric Mach-Zehnder interferometer for analyzing the relative phase of the double optical pulse and an optical system for analyzing the arrival time of the optical pulse without using a phase modulator. Are prepared in parallel so that the light pulses transmitted from the transmitter are incident on each analysis system with equal probability. One analysis system can make measurements on the Y basis and the other analysis system can make measurements on the Z basis. Each operation will be described below.

  When a double light pulse belonging to the Y-base is incident on the asymmetric Mach-Zehnder interferometer, the output is a triple light pulse, and the central part is a superposition of the double light pulses before incidence. . Since the relative phase of the double light pulse is + 90 ° in the first state and −90 ° in the second state, the phase difference between the long optical path and the short optical path of the asymmetric Mach-Zehnder interferometer is + 90 °. As a result, the relative phase of the double light pulse can be set to 180 ° (reverse phase) in the first state and 0 ° (in-phase) in the second state. Accordingly, the two states of the Y base can be identified depending on which one of the two output ports of the asymmetric Mach-Zehnder interferometer is detected.

  Naturally, it is possible to identify the case where an optical pulse belonging to the Z base is incident on the optical system for analyzing the arrival time of the optical pulse. This is because the two states of the Z base are a state in which the optical pulse is in the previous time slot or the subsequent time slot, and the arrival times are different.

  The four states belonging to the two non-orthogonal bases can be generated on the transmitting device side by the principle and method as described above, and the two states can be identified on each base on the receiving device side. Therefore, it is possible to execute the BB84 protocol using the present invention and securely share the encryption key.

[Configuration of the embodiment]
FIG. 1 is a diagram showing a configuration of an embodiment of a quantum key distribution system according to the present invention. A transmission device (Alice) 101 and a reception device (Bob) 102 are connected by an optical fiber communication path 103.

  The configuration of the transmission apparatus 101 is as follows. The output of the weak laser light source 104 built in the transmission apparatus 101 is input to an optical delay circuit 105 such as an asymmetric Mach-Zehnder interferometer, and then the output is two phase modulators (first and second) connected in parallel. Phase modulators) 106-1 and 106-2. The two phase modulators 106-1 and 106-2 connected in parallel are branched into two optical paths by a 50/50 coupler (first coupler) 106-3 or the like on the input side thereof, one for each optical path. After passing through the first and second phase modulators 106-1 and 106-2 arranged one by one, the two optical paths are again coupled by a coupler (second coupler) 106-4 and output. This output is connected to the optical fiber communication path 103 as an output from the transmission apparatus 101.

  Next, the configuration of the receiving apparatus 102 will be described. Input light from the optical fiber communication path 103 is input to the receiving apparatus 102 and branched into two optical paths by an internal 50/50 coupler 111 or the like. One is connected to the asymmetric Mach-Zehnder interferometer 107, the other is connected to the optical system 108 for analyzing the arrival time of the optical pulse, and the photon detector 109 is connected to each output port.

  In place of the configuration of the receiving apparatus 102 shown in FIG. 1, an optical system for analyzing the arrival time of an optical pulse by connecting a 50/50 coupler 202 inside an asymmetric Mach-Zehnder interferometer 201 as shown in FIG. Even when branched to 203, the receiving apparatus 102 ′ has the same function as the receiving apparatus 102 shown in FIG.

[Operation]
The light pulse generated from the weak laser light source 104 built in the transmission device 101 enters the optical delay circuit 105, and passes through a long optical path having a different optical path length and a short optical path so that they are combined again. Become. Two phase modulators 106-1 and 106-2 connected in parallel are used with respect to this double light pulse, and one phase modulator 106-1 uses binary phase modulation of {0 °, 90 °}. And the other phase modulator 106-2 provides binary phase modulation of {0 °, 180 °}. As described in the “Concept of the Invention” section, these two phase modulators 106-1 and 106-2 can generate the four states 110 required for the BB84 protocol. The optical pulse generated in this way is transmitted to the receiving device 102 through the optical fiber communication path 103.

  In the receiving apparatus 102, the transmitted optical pulse is branched by a 50/50 coupler 111 or the like, and is incident on an asymmetric Mach-Zehnder interferometer 107 and an optical system 108 for analyzing the arrival time of the optical pulse, respectively. As described in the section [Concept of the Invention], the asymmetric Mach-Zehnder interferometer 107 can identify two states belonging to the Y base, so if the transmitted light pulse belongs to the Y base, it is received. Device 102 can thereby determine whether the bit is 0 or 1. Conversely, since the optical system 108 that analyzes the arrival time can identify two states belonging to the Z base, if the transmitted optical pulse belongs to the Z base, the receiving apparatus 102 thereby sets the bit to 0. Or 1 can be determined.

  It should be noted that a photon is detected by the optical system 108 that analyzes the arrival time even though the state of the Y base is transmitted, or asymmetric Mach-Zehnder interference, although the opposite Z base is transmitted. If the photon is detected by the total 107, the receiving apparatus 102 will obtain a random measurement result, and the transmitted bit value cannot be estimated. For this reason, the BB84 protocol includes a process in which the base used by both parties is disclosed via a normal communication path (not shown) after the transmission / reception of the optical pulse is completed, and only the event due to the optical pulse with the matching base is extracted. (In this case, since the bit value is not disclosed and only the base is disclosed, this communication may be wiretapped). By performing the processing as described above, the encryption key can be securely shared.

FIG. 1 is a diagram showing a configuration of an embodiment of a quantum key distribution system according to the present invention. FIG. 2 is a diagram showing another configuration example of the receiving apparatus shown in FIG. FIG. 3 is a configuration diagram of a first example of a conventional quantum key distribution system. FIG. 4 is a configuration diagram of a second example of a conventional quantum key distribution system.

Explanation of symbols

DESCRIPTION OF SYMBOLS 101 Transmission apparatus 102 Reception apparatus 103 Optical fiber communication path 104 Weak laser light source 105 Optical delay circuit 106-1 and 106-2 Phase modulator connected in parallel 107 Asymmetric Mach-Zehnder interferometer 108 Optical system which analyzes arrival time of optical pulse 109 Photon Detector 110 Four States Required for BB84 Protocol 111 50/50 Coupler 201 Asymmetric Mach-Zehnder Interferometer 202 50/50 Coupler 203 Optical System for Analyzing Arrival Time of Optical Pulse

Claims (6)

A laser light source that generates an optical pulse; an optical delay circuit that receives the optical pulse to generate a coherent duplex optical pulse; a first coupler that branches an optical path from the optical delay circuit into two; A first and a second phase modulator connected to each output port of one coupler, and a second coupler for coupling the outputs of the first and second phase modulators into one optical path; The first phase modulator performs phase modulation of {0 °, 90 °} on the incident optical pulse, and the second phase modulator applies phase of {0 °, 180 °} to the incident optical pulse. a transmission apparatus in combination with receiving apparatus used for row cormorants quantum key distribution system encoded by performing modulation,
An optical delay circuit that receives an optical pulse from the transmitter and analyzes its phase difference, and an optical system that analyzes the arrival time of the optical pulse,
The optical pulse from the transmission device is branched into two by a coupler, and one branched optical pulse is sent to the asymmetric Mach-Zehnder interferometer as the optical delay circuit on the receiving device side, and the other branched optical pulse is sent to the optical device. A receiving apparatus characterized by being incident on each system . A laser light source that generates an optical pulse; an optical delay circuit that receives the optical pulse to generate a coherent duplex optical pulse; a first coupler that branches an optical path from the optical delay circuit into two; A first and a second phase modulator connected to each output port of one coupler, and a second coupler for coupling the outputs of the first and second phase modulators into one optical path; The first phase modulator performs phase modulation of {0 °, 90 °} on the incident optical pulse, and the second phase modulator applies phase of {0 °, 180 °} to the incident optical pulse. A reception device used in combination with a transmission device for a quantum key distribution system that performs encoding by performing modulation,
An optical delay circuit that receives an optical pulse from the transmitter and analyzes its phase difference, and an optical system that analyzes the arrival time of the optical pulse,
An optical pulse from the transmitting device is incident on an asymmetric Mach-Zehnder interferometer as the optical delay circuit on the receiving device side, and a branched optical pulse is connected by connecting a branching coupler inside the asymmetric Mach-Zehnder interferometer. rECEIVER you wherein a to be incident on the optical system. Quantum key distribution system characterized in that it comprises a transmitting device and a receiving equipment according to claim 1. A quantum key distribution system comprising the transmission device and the reception device according to claim 2. The transmitter is
By using the first and second phase modulators connected in parallel and individually modulating each optical pulse of the branched duplex optical pulse,
The first phase modulator modulates a first phase shift amount to each of the branched duplex optical pulses, and the second phase modulator applies a first to one of the branched duplex optical pulses. The first state is generated by modulating the phase shift amount and modulating the other pulse by π larger than the first phase shift amount , or
The first phase modulator modulates a second phase shift amount to each of the branched duplex optical pulses, and the second phase modulator applies a second to one of the branched duplex optical pulses. The second state is generated by modulating the phase shift amount by π larger than the phase shift amount, and the second phase shift amount is modulated on the other pulse, or the first phase modulator generates the branched duplex optical pulse. each performs modulation of the third phase shift amount, said second phase modulator performs a third phase shift amount than [pi / 2 small modulation to one pulse of two consecutive light pulses branched, other pulse To generate a third state by modulating the phase shift amount by π / 2 larger than the third phase shift amount , or the first phase modulator modulates the fourth phase shift amount to each of the branched optical pulses. And the second phase modulator is divided into two series. Perform a fourth phase shift amount than [pi / 2 larger modulation to one pulse of the pulse, and wherein generating a fourth state in the fourth to perform the phase shift amount than [pi / 2 small modulation to the other pulse The quantum key distribution system according to claim 3 or 4 . The quantum key distribution system according to claim 3, further comprising an asymmetric Mach-Zehnder interferometer as the optical delay circuit in the transmission device .

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